Load transfer devices have numerous applications, for example in building, mining and civil engineering for transferring loads along an elongate overhead guide such as a cable, rod or track. Such arrangements may also find use in transferring goods and/or personnel from ship to shore and vice versa at quayside locations.
Some known load transfer devices suffer from the drawback that they are incapable of negotiating the intermediate brackets along the elongate support element. One solution to this problem is to provide special brackets which can be "opened" to allow the supported load to pass. The weakness of this approach is that the elongate support element temporarily lacks support at the very point where the installer thought it necessary and at the precise moment when it is most needed. Another likely problem is that the brackets may not be accessible to the system user.
An alternative solution is to employ special entry/exit fittings or access points along the elongate support element so that the load transfer device can be attached and removed. The drawback of this proposal is that the access points are not always conveniently situated in relation to the exact location at which attachment or removal is desired.
Improved load transfer devices have been developed which are capable of automatically traversing intermediate brackets for the elongate support element without user intervention. Such devices typically comprise a pair of rotatable wheels having a series of recesses at spaced locations around their peripheries, the adjacent recesses being separated by a radially projecting part of the wheel. A cooperating slipper part is mounted on the wheels by means of formations which inter-engage with complementary formations on the radially projecting wheel parts. A space between the slipper part and the wheels is dimensioned to receive an elongate support element such as a cable or a rigid elongate element.
In use, the device is able to negotiate intermediate brackets for the elongate support element without user intervention by accommodating the bracket legs in a pair of aligned recesses carried by the respective wheels. Rotation of the wheels relative to the slipper part causes the intermediate bracket to pass behind the slipper part, in the aligned recesses of the rotating wheels. Examples of such devices are described In the Applicant's British Patent No. 2 096 958 and in International Patent Application No. WO96/02456.
Similarly, vertical fall arrest devices are an important accessory for maintenance personnel who climb tall structures, since they enable the hazard of falls to be minimised. Vertical fall arrest systems which employ a safety line such as a flexible cable for engagement by the fall arrest device require intermediate support brackets to restrain the cable from buffeting against the tall structure while under wind loading. Such systems therefore present a practical problem of enabling the fall arrest device (and the user) to bypass the support brackets without increasing the fall hazard.
Certain known designs attempt to overcome this bypass problem by using a manually operated bracket lock. This requires the user to open and close the bracket when he traverses it. Other known designs require that the user should lean out from the normal climb/descend posture and pull the cable away from the bracket in order to move the fall arrest device past the bracket position. Both of these methods add significantly to the difficulty of the climb, are more tiring and hence possibly increase the fall hazard.
Another problem facing maintenance personnel on very tall structures such as telecommunication pylons, masts etc. is the provision of a number of discrete vertical fall arrest systems up the side of the structure. This is due to the fact that ladder placement is often along a number of different climbing axes. Such structures may therefore require the detachment and re-attachment of the fall arrest device at any point during the climb or descent, and the ease by which this can be achieved is an important factor in determining the overall safety of the maneuver.
Examples of a vertical fall arrest devices that address the aforementioned problems are described in the Applicant's International Patent Application Nos.
WO95/26784 and WO96/09089. These devices work on a similar principle to the known load transfer devices described above, in that intermediate support brackets are traversed by accommodating them in radially-disposed notches provided in rotatable wheels forming an essential feature of the fall arrest device. The rims of said wheels are enclosed behind a so-called "slipper" member, this inter-engagement serving to prevent undesired disengagement of the fall arrest device from the elongate support element.
One of the drawbacks of known fall arrest systems has already been mentioned above: Very tall structures are often provided with a number of discrete vertical fall arrest systems at different locations around the periphery of the structure, ladder placement being along a number of different climbing axes. Hence, maintenance personnel must detach themselves from one vertical fall arrest system and undertake a horizontal traverse, perhaps unsecured, before attaching themselves to the next span of vertical safety line.
Another disadvantage is that limitations arise in passing support brackets, particularly when negotiating corners. This problem is made more difficult to overcome when the user is not optimally aligned with the intermediate support brackets, for example when operator freedom is restricted by narrow walkways or when equipment orientation is hindered by restricted wire positions.
Presently-known systems are usually designed around an elongate wire or rod element, or a rigid track profile. Both of these have their own inherent advantages and limitations. A wire is usually preferred for long systems and where few intermediate anchor and/or support positions are available. Such systems are also simple in design, relatively cheap to install and hence cost-effective. However, a wire has the disadvantage that it must be supported by an element such as a loop which at least partially surrounds the wire. Hence, the running surface is periodically interrupted and special provisions must be made for negotiation of intermediate supports, as discussed above. A rigid track is better suited to a situation where there are numerous intermediate support and fixing points. Track has advantages for relatively heavy loads and may be better suited to systems subjected to frequent use. Obviously, rigid tracks do not allow such a flexible approach to installation and are Inherently more costly, particularly if building modifications are required to install sub-structures for supporting track.
A track system is known from French Patent Application No. 2 681 253 which discloses at least one profiled rail suspended from supports. The rail Is equipped with pre-stressing means and has a running surface that is adapted to receive at least one trolley In a freely sliding manner. The trolley has means for receiving a hook such as a karabiner clip which is used to link the device to the personal safety harness of a user.